Oxide based memory

ABSTRACT

Methods, devices, and systems associated with oxide based memory are described herein. In one or more embodiments, a method of forming an oxide based memory cell includes forming a first electrode, forming a tunnel barrier, wherein a first portion of the tunnel barrier includes a first material and a second portion of the tunnel barrier includes a second material, forming an oxygen source, and forming a second electrode.

PRIORITY INFORMATION

This application is a Divisional of U.S. application Ser. No.12/794,430, filed Jun. 4, 2010, which is incorporated by reference.

TECHNICAL FIELD

The present disclosure relates generally to semiconductor memory devicesand methods, and more particularly, to oxide based memory devices andmethods.

BACKGROUND

Oxide based memory devices can be used in a number of electrical systemsand can include two electrodes with a tunnel barrier and an oxygensource between the two electrodes. Oxide based memory devices canoperate by applying a positive voltage across the electrodes of thememory device to cause oxygen ions from the oxygen source to move to thetunnel barrier. A negative voltage can be applied across the electrodesof the memory device to cause oxygen ions to move from the tunnelbarrier to the oxygen source. The resistivity of the memory device isdependent on the oxygen ion location and changes as the location of theoxygen ions changes, either in the tunnel barrier, in the oxygen sourceand/or in a portion of each. Therefore, the state of the memory devicecan change depending on the location of the oxygen ions and the state ofthe device can be read by applying a read voltage across the electrodesof the memory device.

In oxide based memory devices, the retention of the oxygen ions in thetunnel barrier and/or oxygen source can determine the ability of thememory device to maintain a state. For memory devices that better retainoxygen ions in the tunnel barrier it can also be more difficult to movethe oxygen ions to the tunnel barrier, e.g., more current is required.And memory devices that can more easily move oxygen ions to the tunnelbarrier, e.g., less current is required, may not satisfactorily retainthe oxygen ions in the tunnel barrier. A memory device that exhibitspoor retention of the oxygen ions in the tunnel barrier may not bereliable in maintaining an “on” state or an “off” state for the memorydevice. An oxide based memory device that can move oxygen ions betweenthe oxygen source and the tunnel barrier and retain the oxygen ions inthe tunnel barrier may have greater reliability in maintaining an “on”state or an “off” state.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross-sectional view of a portion of an oxide basedmemory cell formed in accordance with one or more embodiments of thepresent disclosure.

FIG. 2 illustrates a cross-sectional view of a portion of an oxide basedmemory cell formed in accordance with one or more embodiments of thepresent disclosure.

FIG. 3 illustrates a cross-sectional view of a portion of an oxide basedmemory cell formed in accordance with one or more embodiments of thepresent disclosure.

FIG. 4 is a graphical illustration of the energy band diagram for anoxide based memory cell formed in accordance with one or moreembodiments of the present disclosure.

FIG. 5 is a graphical illustration of the energy band diagram for anoxide based memory cell formed in accordance with one or moreembodiments of the present disclosure.

FIG. 6 is a graphical illustration of the energy band diagram for anoxide based memory cell formed in accordance with one or moreembodiments of the present disclosure.

FIG. 7 is a table that illustrates the currents and current ratios in aprior art oxide based memory cell and an oxide based memory cell formedin accordance with one or more embodiments of the present disclosurewhen biased at 0.5 volts (V).

DETAILED DESCRIPTION

Methods, devices, and systems associated with oxide based memory aredescribed herein. In one or more embodiments, a method of forming anoxide based memory cell includes forming a first electrode, forming atunnel barrier, wherein a first portion of the tunnel barrier includes afirst material and a second portion of the tunnel barrier includes asecond material, forming an oxygen source, and forming a secondelectrode.

One or more embodiments of the present disclosure can provide benefitssuch as reducing the current used for a write operation and the currentused for an erase operation. For instance, one or more embodiments canprovide the ability to control the current used, and thus voltage, tomove oxygen ions to a tunnel barrier, to retain the oxygen ions in thetunnel barrier, and to control the current used, and thus voltage, tomove the oxygen ions from the tunnel barrier to the oxygen source. Theability to move oxygen ions to and/or retain oxygen ions in the tunnelbarrier in a programmed state can help provide a stable and reliablememory device.

Embodiments of the present disclosure will now be described in detailwith reference to the accompanying figures. It should be noted thatalthough the figures illustrate only one memory cell, the semiconductorstructures contemplated herein can have more than one memory cell.

In the following detailed description of the present disclosure,reference is made to the accompanying drawings that form a part hereof,and in which is shown by way of illustration how one or more embodimentsof the disclosure may be practiced. These embodiments are described insufficient detail to enable those of ordinary skill in the art topractice the embodiments of this disclosure, and it is to be understoodthat other embodiments may be utilized and that process, electrical,and/or structural changes may be made without departing from the scopeof the present disclosure.

The figures herein follow a numbering convention in which the firstdigit or digits correspond to the drawing figure number and theremaining digits identify an element or component in the drawing.Similar elements or components between different figures may beidentified by the use of similar digits. For example, 106 may referenceelement “06” in FIG. 1, and a similar element may be referenced as 206in FIG. 2. As will be appreciated, elements shown in the variousembodiments herein can be added, exchanged, and/or eliminated so as toprovide a number of additional embodiments of the present disclosure. Inaddition, the proportion and the relative scale of the elements providedin the figures are intended to illustrate various embodiments of thepresent invention and are not to be used in a limiting sense.

FIG. 1 illustrates a cross-sectional view of a portion of an oxide basedmemory cell formed in accordance with one or more embodiments of thepresent disclosure. The cell 100 includes a bottom electrode 102 formedover a substrate 110. The substrate 110 can be a silicon substrate orSOI substrate, among others.

In one or more embodiments, the bottom electrode 102 can be any suitablemetal, such as platinum (Pt), among other metals. The bottom electrodecan be formed by deposition processes such as, but not limited to,chemical vapor deposition (CVD) and plasma vapor deposition (PVD), etc.The bottom electrode can be coupled to circuitry that can providecurrent to the memory cell for programming, erasing, and/or reading thememory cell.

In one or more embodiments, tunnel barrier 106 can be formed over thebottom electrode 102. A first portion of the tunnel barrier 106-1 caninclude a first material starting a first edge 112 of the tunnel barrier106 and a second portion of the tunnel barrier 106-2 can include asecond material ending at a second edge 114 of the tunnel barrier 106.In one or more embodiments, the first material can be aluminum oxide(AlO_(x)), silicon dioxide (SiO₂), silicon oxynitride (SiON), hafniumsilicon oxide (HfSiO_(x)), hafnium silicon nitride (HfSiN), zirconiumsilicon oxide (ZrSiO_(x)), or zirconium silicon oxynitride (ZrSiON),among other materials, and the second material can be hafnium oxide(HfO_(x)), zirconium oxide (ZrO_(x)), titanium oxide (TiO₂), hafniumzirconium oxide (HfZrO_(x)), hafnium titanium oxide (HfTiO_(x)),zirconium titanium oxide (ZrTiO_(x)), or strontium oxide (SrO), amongother materials. The first material and the second material can beformed using atomic layer deposition (ALD), chemical vapor deposition(CVD), and/or sputtering, among other suitable formation techniques.

In one or more embodiments, oxygen source 108 can be formed over thetunnel barrier 106. The oxygen source 108 can be formed of perovskitemetal oxide (PCMO), titanium oxide (TiO_(x)), and/or magnesium oxide(MgO), among other suitable oxygen sources. The oxygen source 108 canprovide the source of the oxygen ions that move from the oxygen source108 to the tunnel barrier 106 when a biasing voltage is applied to thememory cell 100 that is sufficient to cause the oxygen ions to move fromthe oxygen source 108 to the tunnel barrier 106. The oxygen source canhave a thickness of approximately 20-2000 angstroms (Å).

As discussed herein, the tunnel barrier 106 of the present disclosurecan form an oxygen ion trapping area of a memory cell 100. In someembodiments, the tunnel barrier 106 can have a size in the range ofapproximately two (2) to one hundred (100) nanometer (nm) in thickness.

The cell 100 includes a top electrode 104 formed over oxygen source 108.In one or more embodiments, the top electrode 104 can be any suitablemetal, such as platinum (Pt), among other metals. The top electrode canbe formed by deposition processes such as, but not limited to, chemicalvapor deposition (CVD) and/or plasma vapor deposition (PVD), etc. Thetop electrode can be coupled to circuitry that can provide current tothe memory cell for programming, erasing, and/or reading the memorycell.

In one or more embodiments, the tunnel barrier 106 can be formed from afirst material and a second material. The first portion of the tunnelbarrier 106-1, which includes the first material, can provide anactivation energy for oxygen ion diffusion that is higher than thesecond material's activation energy for oxygen ion diffusion and is morelikely to retain oxygen ions that enter the first material once thebiasing voltage is no longer applied to the memory cell. The secondportion of the tunnel barrier 106-2, which includes the second material,can provide an activation energy for oxygen ion diffusion that is lowerthan first material's activation energy for oxygen ion diffusion andallows the oxygen ions to move from the oxygen source 108 to and throughthe second portion of tunnel barrier 106-2 at a lower biasing voltagethan the first portion of the tunnel barrier 106-1, which includes thefirst material. The combination of the first material and the secondmaterial in the tunnel barrier provide for the combined properties ofease of transfer of the oxygen ions to the tunnel barrier and retentionof the oxygen ions in the tunnel barrier.

The memory cell 100 can use the tunnel barrier 106 and the oxygen source108 as part of a multi-resistive memory cell. Accordingly, memorydevices of an embodiment of the present disclosure are suitable as anon-volatile memory and can be scaled for future technologies. Thememory devices can be used in a memory array that uses cross pointarchitecture, for example, among other suitable architectures. A memorydevice according to the present disclosure can change resistance moreeasily and be more reliable in maintaining a resistive state.

FIG. 2 illustrates a cross-sectional view of a portion of an oxide basedmemory cell formed in accordance with one or more embodiments of thepresent disclosure. The cell 200 includes a bottom electrode 202 formedover a substrate 210, such as described above in association with FIG.1.

In one or more embodiments, tunnel barrier 206 can be formed over thebottom electrode 202. In one or more embodiments, a first portion of thetunnel barrier 206-1 can include a first material, a second portion ofthe tunnel barrier 206-2 can include a second material, a third portionof the tunnel barrier 206-3 can include a third material, and a fourthportion of the tunnel barrier 206-4 can include a fourth material. Invarious embodiments, the thickness of each portion of the tunnel barriercan vary. In one or more embodiments, the first and third materials canbe aluminum oxide (AlO_(x)), silicon dioxide (SiO₂), silicon oxynitride(SiON), hafnium silicon oxide (HfSiO_(x)), hafnium silicon nitride(HfSiN), zirconium silicon oxide (ZrSiO_(x)), or zirconium siliconoxynitride (ZrSiON), among other materials, and the second and fourthmaterials can be hafnium oxide (HfO_(x)), zirconium oxide (ZrO_(x)),titanium oxide (TiO₂), hafnium zirconium oxide (HfZrO_(x)), hafniumtitanium oxide (HfTiO_(x)), zirconium titanium oxide (ZrTiO_(x)), orstrontium oxide (SrO), among other materials. In various embodiments,the tunnel barrier can include any number of portions which alternatebetween the first or third materials and the second or fourth materials.Other combinations can be provided in various embodiments of the presentdisclosure. The thickness and proportions of the materials forming thetunnel barrier 206 can be modified to have desired operatingcharacteristics, such as a desired programming and/or erase current. Thefirst, second, third, and fourth materials can be formed using atomiclayer deposition (ALD), chemical vapor deposition (CVD), and/orsputtering, among other suitable formation techniques.

In one or more embodiments, oxygen source 208 can be formed over thetunnel barrier 206. The oxygen source 208 can be formed of perovskitemetal oxide (PCMO), titanium oxide (TiO_(x)), and/or magnesium oxide(MgO), among other suitable oxygen sources. The oxygen source 208 canprovide the source of the oxygen ions that move from the oxygen source208 to the tunnel barrier 206 when a biasing voltage is applied to thememory cell 200 that is sufficient to cause the oxygen ions to move fromthe oxygen source 208 to the tunnel barrier 206.

The cell 200 includes a top electrode 204 formed over oxygen source 208,such as described above in association with FIG. 1. The top electrodecan be coupled to circuitry that can provide current to the memory cellfor programming, erasing, and/or reading the memory cell.

FIG. 3 illustrates a cross-sectional view of a portion of an oxide basedmemory cell formed in accordance with one or more embodiments of thepresent disclosure. The cell 300 includes a bottom electrode 302 formedover a substrate 310, such as described above in association with FIG.1.

In one or more embodiments, tunnel barrier 306 can be formed over thebottom electrode 302. The tunnel barrier 306 can include a number ofmaterials, such as a first material that can be aluminum oxide(AlO_(x)), silicon dioxide (SiO₂), silicon oxynitride (SiON), hafniumsilicon oxide (HfSiO_(x)), hafnium silicon nitride (HfSiN), zirconiumsilicon oxide (ZrSiO_(x)), or zirconium silicon oxynitride (ZrSiON),among other materials, and a second material that can be hafnium oxide(HfO_(x)), zirconium oxide (ZrO_(x)), titanium oxide (TiO₂), hafniumzirconium oxide (HfZrO_(x)), hafnium titanium oxide (HfTiO_(x)),zirconium titanium oxide (ZrTiO_(x)), or strontium oxide (SrO), amongother materials. In some embodiments, the tunnel barrier 306 can be agraded tunnel barrier including a first material and a second material.The proportion of the first material and the second material can startat 100% of the first material at a first edge 312 of the tunnel barrier.The proportion of the second material increases throughout the tunnelbarrier until the tunnel barrier is 100% of the second material at asecond edge 314 of the tunnel barrier. The second material can beintroduced by diffusing the second material into the first material tocreate the graded tunnel barrier. The first material and the secondmaterial can be formed using atomic layer deposition (ALD), chemicalvapor deposition (CVD), and/or sputtering, among other suitableformation techniques.

In one or more embodiments, oxygen source 308 can be formed over thetunnel barrier 306. The oxygen source 308 can be formed of perovskitemetal oxide (PCMO), titanium oxide (TiO_(x)), and/or magnesium oxide(MgO), among other suitable oxygen sources. The oxygen source 308 canprovide the source of the oxygen ions that move from the oxygen source308 to the tunnel barrier 306 when a biasing voltage is applied to thememory cell 300 that is sufficient to cause the oxygen ions to move fromthe oxygen source 308 to the tunnel barrier 306.

The cell 300 includes a top electrode 304 formed over oxygen source 308,such as described above in association with FIG. 1. The top electrodecan be coupled to circuitry that can provide current to the memory cellfor programming, erasing, and/or reading the memory cell.

In one or more embodiments, the combination of the first material andthe second material in the tunnel barrier provide for the combinedproperties of ease of transfer of the oxygen ions to the tunnel barrierand retention of the oxygen ions in the tunnel barrier. The proportionsof the oxides forming the tunnel barrier can be modified to have desiredoperating characteristics, such as a desired programming or erasecurrent.

FIG. 4 is a graphical illustration of the energy band diagram for anoxide based memory cell formed in accordance with one or moreembodiments of the present disclosure. FIG. 4 illustrates the energyband diagram for the tunnel barrier configuration shown in FIG. 1. Thegreater the activation energy for oxygen ion diffusion, the more likelya portion of the tunnel barrier is to retain oxygen ions once oxygenions move to that portion of the tunnel barrier. In FIG. 4, oxygen ionscan move more easily to the second portion 406-2 of the tunnel barrierthan to the first portion 406-1 of the tunnel barrier, but are lesslikely to be retained in the second portion 406-2 of the tunnel barrierafter the biasing voltage is removed.

In one or more embodiments, the first portion 406-1 of the tunnelbarrier that includes the first material, such as aluminum oxide, has anactivation energy for oxygen ion diffusion of approximately 5 electronvolts (eV) and the second portion 406-2 of the tunnel barrier thatincludes the second material, such as hafnium oxide, has an activationenergy for oxygen ion diffusion of approximately 2.5 eV. The differencesin the activation energies of the first portion 406-1 and the secondportion 406-2 of the tunnel barrier can affect the biasing voltage usedto move oxygen ions to the tunnel barrier and can also affect theability of the tunnel barrier to retain the oxygen ions after thebiasing voltage is removed from the memory cell.

FIG. 5 is a graphical illustration of the energy band diagram for anoxide based memory cell formed in accordance with one or moreembodiments of the present disclosure. FIG. 5 illustrates the energyband diagram for the tunnel barrier configuration shown in FIG. 2. Thedifference in the activation energies indicates that a greater biasingvoltage is required to move oxygen ions from the oxygen source 508through the first and third portions 506-1, 506-3 of the tunnel barrierthan is required to move the oxygen ions from the oxygen source 508through the second and fourth portions 506-2, 506-4 of the tunnelbarrier. In FIG. 5, oxygen ions can move more easily to the second andfourth portions 506-2, 506-4 of the tunnel barrier than to the first andthird portions 506-1, 506-3 of the tunnel barrier, but are less likelyto be retained in the second and fourth portions 506-2, 506-4 after thebiasing voltage is removed.

The differences between the activation energies of the first and thirdportions 506-1, 506-3 and the second and fourth portions 506-2, 506-4 ofthe tunnel barrier can affect the biasing voltage used to move oxygenions to the tunnel barrier and can also affect the ability of the tunnelbarrier to retain the oxygen ions after the biasing voltage is removedfrom the memory cell.

The energy band diagram of FIG. 5 for the tunnel barrier configurationshown in FIG. 2 includes alternating portions of a first material and asecond material, where the second material has an activation energy foroxygen ion diffusion that is less then the activation energy for oxygenion diffusion of the first material. The alternating layers can provideportions where oxygen ions can diffuse more easily, e.g., the second andfourth portions 506-2, 506-4 that include the second material, andportions where oxygen ions are more likely to stay trapped within, e.g.the first and third portions 506-1, 506-3 that include the firstmaterial.

FIG. 6 is a graphical illustration of the energy band diagram for anoxide based memory cell formed in accordance with one or moreembodiments of the present disclosure. FIG. 6 illustrates the energyband diagram for the tunnel barrier configuration shown in FIG. 3. Inthe embodiment of FIG. 6, the tunnel barrier includes a first materialand a second in varying proportions from a first edge 612 to a secondedge 614 of the tunnel barrier 606. The tunnel barrier 606 includes 100%of the first material, which has a higher activation energy for oxygenion diffusion than the second material, at the first edge 612 and 100%of the second material, which as has a lower activation energy foroxygen ion diffusion than the first material, at the second edge 614 ofthe tunnel barrier 606. The tunnel barrier 606 has a decreasing amountof the first material and an increasing amount of the second materialfrom the first edge 612 to the second edge 614 of the tunnel barrier,therefore the activation energy for oxygen ion diffusion decreases fromthe first edge 612 to the second edge 614 of the tunnel barrier 606, asindicated in FIG. 6.

The tunnel barrier 606 includes an activation energy for oxygen iondiffusion that gradually decreases from a first edge 612 of the tunnelbarrier to the other. The curved slope of the tunnel barrier'sactivation energy for oxygen ion diffusion is caused by the proportionof the materials with differing conduction band energies forming thetunnel barrier gradually changing, with a greater proportion of thefirst material at a first edge 612 gradually changing to a greaterproportion of the second material at the second edge 614. The higher theactivation energy for oxygen ion diffusion at the first edge 612 of thetunnel barrier 606 indicates that a greater biasing voltage will be usedto move oxygen ions from the oxygen source 608 to the first edge 612 ofthe tunnel barrier 606. This is because of the larger proportion of thefirst material at the first edge 612 of the tunnel barrier. A lowerbiasing voltage is used to move the oxygen ions from the oxygen source608 to the second edge 614 of the tunnel barrier 606 with a largerproportion of lower activation energy for oxygen ion diffusion secondoxide.

In one or more embodiments, the tunnel barrier 606 has an activationenergy for oxygen ion diffusion of approximately 5 eV at the first edge612 and an activation energy for oxygen ion diffusion of approximately 1eV at the second edge 614. The differences in the activation energies ofthe first edge 612 and the second edge 614 of the tunnel barrier 606 canaffect the biasing voltage used to move oxygen ions to the tunnelbarrier and can also affect the ability of the tunnel barrier to retainthe oxygen ions after the biasing voltage is removed from the memorycell.

FIG. 7 is a table that illustrates the currents and current ratios in aprior art oxide based memory cell 730 and an oxide based memory cellformed in accordance with one or more embodiments of the presentdisclosure 740 when biased at 0.5 volts (V). In FIG. 7, a memory cellaccording to the prior art 730 and a memory cell in accordance with oneor more embodiments of the present disclosure 740 has a potential of 0.5V applied across the cell. The table in FIG. 7 illustrates the currentin the tunnel barrier in nanoamps (nA) for various levels of oxygencharge 720 (Q_(ox)/cm²) in the tunnel barrier.

As shown in FIG. 7, a prior art tunnel barrier that includes hafniumoxide or zirconium oxide has 1.3 nA of current when biased at 0.5 V foran “off” state of a memory cell, e.g., an oxygen charge rich state. Atunnel barrier according to the present disclosure that includes anapproximate thickness of 10 angstroms (Å) of aluminum oxide and 20 Å ofhafnium oxide has a 0.87 nA of current when biased at 0.5 V for an “off”state. Also, a prior art tunnel barrier that includes hafnium oxide orzirconium oxide has 15 nA of current when biased at 0.5 volts (V) for an“on” state of a memory cell, e.g., an oxygen charge depleted state. Atunnel barrier according to the present disclosure that includes anapproximate thickness of 10 angstroms (Å) of aluminum oxide and 20 Å ofhafnium oxide has 14.4 nA of current when biased at 0.5 V for an “on”state. Thus the resulting current on/off ratio for the prior art memorycell having a tunnel barrier of hafnium oxide or zirconium oxide is 11.5and the current on/off ratio for the memory cell of the currentdisclosure having a tunnel barrier of aluminum oxide and hafnium oxideis 16.6. Therefore, the memory cell according the present disclosureresults in a higher current ratio allowing for the memory cell to moreeasily switch between the “off” and “on” states, to have a greaterdifference between the “on” and “off” states, and to better retain theoxygen ions in the tunnel barrier while using less current than theprior art memory cell.

Also, in the FIG. 7, the read current for the cells where the oxygencharge is 0 and oxygen ions are not transferred between the oxygensource and the tunnel barrier is less for the memory cell according thepresent disclosure than for the prior art memory cell. The read currentfor the prior art memory cell is 4.2 nA and the read current for thememory cell with 10 angstroms (Å) of aluminum oxide and 20 Å of hafniumoxide is 3.23 nA when the cells are biased at 0.5 V.

Methods, devices, and systems associated with oxide based memory aredescribed herein. In one or more embodiments, a method of forming anoxide based memory cell includes forming a first electrode, forming atunnel barrier, wherein a first portion of the tunnel barrier includes afirst material and a second portion of the tunnel barrier includes asecond material, forming an oxygen source, and forming a secondelectrode.

It will be understood that when an element is referred to as being “on,”“connected to” or “coupled with” another element, it can be directly on,connected, or coupled with the other element or intervening elements maybe present. In contrast, when an element is referred to as being“directly on,” “directly connected to” or “directly coupled with”another element, there are no intervening elements or layers present. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed items.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements and that these elementsshould not be limited by these terms. These terms are only used todistinguish one element from another element. Thus, a first elementcould be termed a second element without departing from the teachings ofthe present disclosure.

Although specific embodiments have been illustrated and describedherein, those of ordinary skill in the art will appreciate that anarrangement calculated to achieve the same results can be substitutedfor the specific embodiments shown. This disclosure is intended to coveradaptations or variations of various embodiments of the presentdisclosure.

It is to be understood that the above description has been made in anillustrative fashion, and not a restrictive one. Combination of theabove embodiments, and other embodiments not specifically describedherein will be apparent to those of skill in the art upon reviewing theabove description. The scope of the various embodiments of the presentdisclosure includes other applications in which the above structures andmethods are used. Therefore, the scope of various embodiments of thepresent disclosure should be determined with reference to the appendedclaims, along with the full range of equivalents to which such claimsare entitled.

In the foregoing Detailed Description, various features are groupedtogether in a single embodiment for the purpose of streamlining thedisclosure. This method of disclosure is not to be interpreted asreflecting an intention that the disclosed embodiments of the presentdisclosure have to use more features than are expressly recited in eachclaim.

Rather, as the following claims reflect, inventive subject matter liesin less than all features of a single disclosed embodiment. Thus, thefollowing claims are hereby incorporated into the Detailed Description,with each claim standing on its own as a separate embodiment.

1-20. (canceled)
 21. A method of fabricating a memory cell, comprising:forming a first electrode; forming a tunnel barrier over the firstelectrode, wherein the tunnel barrier is a graded tunnel barrier thatincludes an aluminum oxide material and a different material; forming anoxygen source over the tunnel barrier; and forming a second electrodeover the oxygen source.
 22. The method of claim 21, wherein forming thetunnel barrier over the first electrode includes forming the gradedtunnel barrier by depositing the aluminum oxide material over the firstelectrode and introducing the different material to the tunnel barrierin increasing proportion until the tunnel barrier is complete and thealuminum oxide material not present at an edge of the tunnel barrieropposite an edge of the tunnel barrier that interfaces with the firstelectrode.
 23. The method of claim 21, wherein forming the tunnelbarrier over the first electrode includes forming the graded tunnelbarrier by depositing the aluminum oxide material over the firstelectrode to have a composition starting as approximately 100% of thealuminum oxide material at an interface of the first electrode and thetunnel barrier and gradually increasing the amount of the secondmaterial in the tunnel barrier throughout the tunnel barrier until thetunnel barrier include approximately 100% of the different material atan interface of the tunnel barrier and the oxygen source.
 24. The methodof claim 21, wherein forming the tunnel barrier over the first electrodeincludes forming the graded tunnel barrier by doping the tunnel barrierwith the different material.
 25. The method of claim 21, wherein formingthe tunnel barrier over the first electrode includes forming the gradedtunnel barrier using chemical vapor deposition (CVD).
 26. The method ofclaim 21, wherein forming the tunnel barrier over the first electrodeincludes forming the graded tunnel barrier using atomic layer deposition(ALD).
 27. A method of forming an oxide based memory cell, comprising:forming a first electrode; forming a tunnel barrier by depositing afirst material and doping the first material of the tunnel barrier witha second material, wherein the first material has a first activationenergy for oxygen ion diffusion and the second material has a secondactivation energy for oxygen ion diffusion; forming an oxygen source;and forming a second electrode.
 28. The method of claim 27, wherein afirst activation energy for oxygen ion diffusion of the first materialis greater than the second activation energy for oxygen ion diffusion ofthe second material.
 29. The method of claim 27, wherein the firstmaterial is aluminum oxide and the second material is hafnium oxide. 30.The method of claim 27, wherein the first material is aluminum oxide andthe second material is zirconium oxide.
 31. The method of claim 27,wherein forming the tunnel barrier includes depositing the firstmaterial over the first electrode to have a composition starting asapproximately 100% of the first material at the interface of the firstelectrode and the tunnel barrier and gradually increasing the amount ofthe second material in the tunnel barrier throughout the tunnel barrieruntil the tunnel barrier include approximately 100% of the secondmaterial at the interface of the tunnel barrier and the oxygen source.32. The method of claim 21, including forming the tunnel barrier usingchemical vapor deposition (CVD).
 33. The method of claim 21, includingforming the tunnel barrier using atomic layer deposition (ALD).
 34. Amemory device, comprising: an array of memory cells; and circuitry forcontrol and/or access of the array; wherein at least one memory cell ofthe array includes: a first electrode; an oxygen source; a tunnelbarrier, wherein the tunnel barrier is a graded tunnel barrier thatincludes a first material and a second material; and a second electrode.35. The memory device of claim 34, wherein the oxygen source includes anumber of oxygen ions.
 36. The memory device of claim 34, wherein aportion of the number of oxygen ions move from the oxygen source to thetunnel barrier when the first electrode is biased positively.
 37. Thememory device of claim 36, wherein the portion of the number of oxygenions are trapped in the tunnel barrier after the first electrode isbiased positively and a voltage potential is no longer present in thememory cell.
 38. The memory device of claim 36, wherein the portion ofthe number of oxygen ions move from the tunnel barrier to the oxygensource when the first electrode is biased negatively.
 39. The memorydevice of claim 34, wherein the first material of the tunnel barrier isadjacent to the first electrode and the second material of the tunnelbarrier is adjacent to the oxygen source.
 40. The memory device of claim34, wherein proportion of the second material in the tunnel barrierincreases from about 0% at the interface of the first electrode and thetunnel barrier to about 100% at the interface of the tunnel barrier andthe oxygen source.